(1) Field of the Invention
The present invention relates to an image forming apparatus such as a printer, and particularly relates to an image forming apparatus that include a mechanism for maintaining a gap between a photosensitive drum and a developing roller at a defined value, and an image forming unit and a developing unit included in the image forming apparatus.
(2) Related Art
An image forming apparatus employing an electronic photography system such as a printer performs image formation by exposure-scanning a circumferential surface of a photosensitive drum to form an electrostatic latent image, supplying toner to the electrostatic latent image for development from a developing device via a developing roller, and transferring a toner image onto a recording sheet.
A developing device employing a two-component developing system, among various types of developing devices, for example has a structure in which a developing roller includes a cylindrical developing sleeve into which a magnet roller is inserted. Generally, a flange having a rotational shaft which is vertically provided on the center thereof is attached to an opening of each of ends of a developing sleeve, and the developing sleeve is pivotally supported by the rotational shaft so as to be rotatable relative to a housing of the developing device.
By the way, in order to excellently form a toner image on a photosensitive drum, it is necessary to maintain a gap between a circumferential surface of a developing sleeve and a circumferential surface of the photosensitive drum at a predetermined defined value with a high precision. Such a gap is hereinafter referred to as a development gap.
For this reason, conventional image forming apparatuses for example have the following structure such as disclosed in Japanese Patent Application Publication No. H11-161015. A developing sleeve has a rotational shaft to which a roller for controlling a gap (hereinafter, DS roller) is attached whose outer diameter is slightly longer than a diameter of the developing sleeve, and a developing roller is forced toward a photosensitive drum to bring a circumferential surface of the DS roller into abutment with a circumferential surface of the photosensitive drum, thereby maintaining a development gap at a defined value.
However, abutment of the DS roller with the photosensitive drum might obstruct smooth rotation of the photosensitive drum due to abrasion of a part of the photosensitive drum which is in abutment with the DS roller, a load torque which prevents rotation of the photosensitive drum, or the like. Since many image forming apparatuses recently have adopted a structure of rotating a developing sleeve in a direction counter to a rotational direction of a photosensitive drum in order to efficiently supply toner to a developing position, the abrasion and load torque as described exercise a significant negative influence on smooth rotation of the photosensitive drum especially in the image forming apparatuses adopting such a structure.
In view of this, there has proposed a structure of bring the DS roller not into direct abutment with the photosensitive drum but into partial abutment with support frames which support ends of the photosensitive drum so as to secure a development gap.
FIG. 15 is a cross-sectional view of a photosensitive unit 500 and a developing unit 503 which adopt the above structure. FIG. 15 shows respective ends of the photosensitive unit 500 and the developing unit 503 at only one side in a longitudinal direction thereof.
As shown in FIG. 15, the developing unit 503 has the structure in which a flange 505 which is integrally formed with a rotational shaft 5051 is attached to each of ends of a developing sleeve 504, the rotational shaft 5051 is held by a slide bearing 507 so as to be rotatable relative to a housing 506, and a DS roller 508 is attached to the end of the rotational shaft 5051.
The housing 506 of the developing unit 503 is forced toward the photosensitive drum 501 by an elastic member which is not illustrated. This brings the DS roller 508 into abutment with a corresponding one of support frames 502 which support both ends of the photosensitive drum 501, thereby maintaining a development gap G to a defined value.
According to the structure such as shown in FIG. 15, since the DS roller 508 is out of contact with the photosensitive drum 501, it is possible to prevent abrasion of the photosensitive drum 501 and decrease of a rotation driving force due to a load torque.
However, this structure requires that the rotational shaft 5051 should have a longer length than a conventional one in order to bring the DS roller 508 into abutment with the support frame 502 which is provided outward from ends of the photosensitive drum 501 in a rotational axis direction of the photosensitive drum 501.
The flange 505 and the rotational shaft 5051, which are integrally formed, are generally formed by cutting and machining a thick round bar of metal so as to secure a certain degree of strength. The longer the rotational shaft 5051 is, the longer a round bar of metal is necessary and the longer a period for cutting and machining the rotational shaft 5051 is also necessary. This causes a problem of increase in manufacturing costs.
In response to this problem, there has proposed a method of assembling the flange 505 and the rotational shaft 5051 as separate parts and then integrating them with each other. However, this method requires an increased number of components, and results in a large tolerance between the components (dimensional tolerance). This might disable to secure the development gap G as the defined value.